The present invention relates to an improvement of a low flow rate moisture feeding system for use especially in the production of semiconductors by the low moisture oxidation technique. More particularly, the present invention relates to the accurately controlled generation and supply of moisture in very small quantities.
In the manufacture of semiconductor elements, the conventional so-called dry oxygen (O2) oxidation technique for coating silicon oxide film by thermal oxidation is now largely replaced by the moisture oxidation technique, which is also called the wet oxygen oxidation method. That is because the silicon oxidation film formed by the moisture oxidation technique is superior to that obtained by the dry oxygen oxidation technique in properties such as insulation strength and masking effect.
Oxide film coating by the aforesaid moisture oxidation technique uses a mixed gas with a moisture content (H2O/O2) of, generally, approximately 20 to 30 percent. The amount of moisture to be mixed into oxygen is approximately 200 to 2000 cubic centimeters in terms of the standard state (xe2x80x9csccmxe2x80x9d). That is, a relatively large quantity of moisture is fed from the reactor for the generation of moisture to the semiconductor manufacturing facilities.
FIG. 6 shows an example of the apparatus for the generation of moisture used in the moisture oxidation technique in current practice. In FIG. 6, H2 represents hydrogen; O2, oxygen; N2, nitrogen gas for purging the system; MFC1 to MFC5, mass controllers; V1 to V5, valves; T1 to T6, thermocouples for measuring the temperature; CV1 to CV5, check valves; F1 to F3, filters; H0 and H1, gas preheater coils; Mx1, oxygen-hydrogen mixer; Mx2, oxygen-moisture mixer; 1, the reactor for the generation of moisture; and SM, processing equipment such as semiconductor manufacturing facilities.
As shown in FIG. 7, the aforesaid reactor 1 for the generation of moisture comprises reactor structural components 2 and 3 provided with a gas supply joint 4 and a moisture gas take-out joint 5, a reflector 9 on the inlet side provide inside the reactor 1 and opposite gas feed passage 4a of the reactor structural component 2, a reflector 12 on the outlet side provided inside the reactor 1 and opposite a moisture gas outlet passage 5a of the reactor structural component 3, a filter 10 provided in the middle of the reactor 1, and a platinum-coated catalyst layer 13 provided on the inside wall of the reactor structural component 3.
The platinum-coated catalyst layer 13, which is formed on the inside wall surface of the reactor structural component 3, is of a double layer construction, having a barrier coat 13a with a platinum coat 13b formed thereupon. The barrier coat 13a is formed of a nitride such as TiN, on which the platinum coat 13b is fixed by a vapor deposition technique or an ion coating technique.
Hydrogen and oxygen are fed into the reactor 1 through the gas feed passage 4a, diffused by gas diffusion means 8 comprising the inlet reflector unit 9, the filter 10, and the outlet reflector unit 12, and then come into contact with the platinum-coated catalyst layer 13. Upon coming into contact with the platinum-coated catalyst layer 13, hydrogen and oxygen are enhanced in reactivity by catalytic action and come to be in what is called the radicalized state. Radicalized, hydrogen and oxygen instantaneously react with each other at a temperature lower than the ignition point to produce moisture (i.e., water) without undergoing combustion at a high temperature.
The flow rates of hydrogen and oxygen which are fed into the reactor 1 are set properly to, for example, 1000 sccm : 600 sccm or so. Generally, a 20 percent oxygen rich material gas mixture of oxygen and hydrogen is sent into the reactor. The gas supply pressure of oxygen and hydrogen is set to 1.0 to 3.0 kg/cm2 to produce approximately 1000 sccm of moisture. The reactor 1 for the generation of
It is noted that the mass flow controllers MFC1 to MFC5 are generally constituted so that the flowing gas reaches a set flow rate as soon as possible. That is, the flow rate of the flowing oxygen or hydrogen gas rises to a set level within approximately one second after the start of the feeding of the gas.
The moisture generator illustrated in FIG. 6 can produce over approximately 1000 sccm of high-purity water. The amount of moisture to be generated and supplied can be controlled relatively easily with high precision by regulating the feeding of oxygen and hydrogen. Thus, the generator is excellent in practical usefulness. However, that moisture generator has problems yet to be solved. Of these, the foremost problem is the control of the flow rate of moisture when it is to be generated in very small quantities.
In recent years, what is called the low moisture oxidation technique is being put to wide practical application in the silicon oxide film coating by moisture oxidation. This low moisture oxidation is practiced using the mixture gas of oxygen and water with a moisture content of 1000 ppm xe2x88x922 percent.
The moisture generator illustrated in FIG. 6, too, is required to regulate the generation of moisture at a very low rate, that is, one to 50 sccm, with high precision. With the moisture generator outlined in FIG. 6, a variety of inconveniences arise and it is virtually impossible to control the generation of moisture at such a low rate, which will be described later.
Shown in FIG. 8 is a testing apparatus developed to test the response characteristics or responsiveness of the reactor 1 for the generation of moisture. Experiments were conducted using this testing apparatus and the response characteristics of the reactor 1 for the generation of moisture were determined with the production of moisture kept at very low levels.
In FIG. 8, MFC1 to MFC3 indicate mass flow controllers; V1 to V6, valves; SV, a suction-regulating valve; E, a quadrupole mass spectrometer (Q-mass spectrometer); P, a vacuum pump (rotary pump); D, a turbo molecular pump; and R, a moisture-collecting reservoir. Moisture is condensed at room temperature, and the condensed moisture is collected. The mass flow controllers MFC1 to MFC3 are moisture is 114 mm in outside diameter, approximately 31 mm in thickness, and 86 cm3 in interior space with 99 cm2 in a platinum-coated catalyst layer area. Though very small in size as shown, this reactor can turn out over 1000 sccm of moisture.
On the outlet side of the reactor is provided the aforementioned oxygen-moisture mixer Mx2 where the moisture as generated can be mixed with oxygen in any desired ratio and diluted.
FIG. 6 illustrates an operation in which a 20 percent oxygen rich material gas mixture is fed into the reactor 1. The reactor 1 can also be operated with a hydrogen-rich material gas mixture. In such an arrangement, a hydrogen-moisture mixer Mx1 is provide instead of the oxygen-moisture mixer Mx2 as necessary.
The aforesaid gas preheater coils H0 and H1 are for heating the material gas mixture or oxygen respectively at not higher than approximately 200xc2x0 C. The reactor 1 is also provided with a heater and, as necessary, a cooler so that if the reaction heat pushes up the temperature in the reactor in operation to over 500xc2x0 C. (which rarely happens), the cooler will be activated to bring down the temperature below 500xc2x0 C. In addition, the mixture in the mixer Mx2 provided near the outlet of the reactor is constantly maintained at approximately 120xc2x0 C. to prevent water from condensing on the pipe wall. A heater is provided as necessary.
Prior to starting up the reactor 1 for the generation of moisture, such equipment as the mass flow controllers MFC1 to MFC5 and temperature controllers first are prepared for operation, and the valves V2 and V5 are opened, and the valves V1, V3, and V4 are closed to purge the system with nitrogen gas. Then the valves V2 and V5 are closed. At the same time or after the lapse of a certain time, V3 and V4 are opened to first feed oxygen into the system. At the same time that oxygen starts to be fed or after a certain time has lapsed after that, V1 is opened to feed hydrogen into the system. In contact with the platinum-coated catalyst layer in the reactor 1, oxygen and hydrogen are radicalized to react with each other to produce approximately 1000 sccm of moisture gas, which is sent out to semiconductor manufacturing facilities SM. the so-called quick-start type mass flow controllers and are designed so that the level of hydrogen and oxygen will reach a specific set flow rate as soon as possible.
For determination of the start-up response characteristics of the reactor 1 for the generation of moisture, the flow rates of hydrogen, oxygen, and nitrogen were first set to the levels in four different cases as shown below by means of the mass flow controllers MFC1 to MFC3. In each case, the concentration of hydrogen, oxygen, and nitrogen in the generated moisture and the generated amount of water were determine using the Q-mass spectrometer E. The valves V1 to V3 were actuated in the following manner: At the start-up of the moisture generator 1, the valve V3 was closed and the valve V2 was opened. Two seconds later, V1 was opened to produce moisture for one minute. When the moisture generation was to be ended, the valve V1 was closed, and two seconds after that, the valve V2 was closed and the valve V3 was opened to feed nitrogen to the reactor 1. Part of the gas flowing out of the reactor 1 was led into the Q-mass spectrometer E and the determination was made of the concentration of hydrogen, oxygen, and nitrogen in the generated moisture and the generated amount of water at a measurement interval of about one second. The Q-mass spectrometer used was the model MSQ-150A Quadrupole Mass Analyzer manufacture by ULVAC CORPORATION of Japan. The supply pressure was set to gauge pressures of 2 kgf/cm2 for hydrogen, 2 kgf/cm2 for oxygen, and 7 kgf/cm2 for nitrogen.
FIG. 9 shows the results of the test carried out by the testing apparatus shown in FIG. 8 to test the moisture generation response characteristics of the prior art moisture generator. As illustrated in FIG. 9, the amount of generated moisture peaked at P in the same in every case. The reason why all four cases 1 to 4 peak at almost the same water level in the moisture generation response characteristics curve may be that some hydrogen remains trapped in the pipe line, the mass flow controller MFC1, the valve V1, and other parts in the hydrogen gas supply system when a cycle is over. Then, when the valve V1 is opened in the next cycle, this remnant hydrogen flows into the reactor 1 and reacts, turning into water, sending the moisture generation to the peak P.
In FIG. 9, the concentration curve of hydrogen peaks at H2xe2x80x2. That is probably because part of the water led into the Q-mass spectrometer E will decompose into H2+ ions in a gas ionizer within the spectrometer E and those ions will be measured together.
In the arrangement of the prior art moisture generator as shown in FIG. 6, the amount of moisture generated is almost the same in the initial stage when the moisture generation is small. In other words, it was shown that it was impossible to control the concentration of water to be mixed, that is, the flow rate of water.
The present invention addresses those problems encountered with the prior art moisture generator. That is, the prior art cannot control the amount of moisture generation or the flow rate of moisture when the moisture generation per unit time is very small, and it is impossible to regulate water in the water mixture gas to be supplied to the respective processes in the semiconductor manufacturing facilities.
It is an object of the present invention to provide a process for feeding moisture at a very low flow rate which permits control with high precision of the flow and feeding of moisture at very low flow rates to a semiconductor manufacturing line.
One embodiment of the present invention provides a process for supplying moisture at a low flow rate for use in a moisture generator in which hydrogen and oxygen are fed into a reactor provided with a platinum coat on the wall in the interior space, enhanced in reactivity by the platinum catalytic action and caused to instantaneously react with each other at a temperature lower than the ignition point to produce moisture without undergoing combustion at a high temperature, characterized in that the flow to the rector of hydrogen is controlled by a flow controller in such a way that while oxygen is supplied to the reactor at a set flow rate, the supply to the reactor of hydrogen is started and gradually increased, reaching a specific set level in a specific time after the start of the feeding of hydrogen, thus producing and supplying moisture or a mixture of moisture with oxygen from the reactor to a semiconductor manufacturing line at a set flow rate.
Another embodiment of the present invention provides a process for supplying moisture at a low flow rate for use in a moisture generator of the same construction as described above, wherein the flow to the reactor of oxygen is controlled by a flow controller in such a way that while hydrogen is supplied to the reactor at a set flow rate, the supply to the reactor of oxygen is started and gradually increased, reaching a specific set level in a specific time after the start of the feeding of oxygen, thus producing and supplying moisture or its mixture with hydrogen from the reactor to a semiconductor manufacturing line at a set flow rate.
Still another embodiment of the present invention provides a process for supplying moisture at a low flow rate as described above, wherein the flow controller is so controlled that the flow of hydrogen or oxygen is raised to a specific set level at a nearly fixed rate of increase wherein time elapsed before the set flow rate level is reached is one to ten seconds.
Still one more embodiment of the present invention provides a process for supplying moisture at a low flow rate for use in a moisture generator in which hydrogen and oxygen are fed into a reactor provided with a platinum coat on the wall in the interior space, enhanced in reactivity by the platinum catalytic action and caused to instantaneously react with each other at a temperature lower than the ignition point to produce moisture without undergoing combustion at a high temperature, characterized in that an escape pipe provided with an escape valve is branched out from the outlet side of the reactor or moisture generator and that, with the escape valve left opened, hydrogen and oxygen are first fed into the reactor at specific rates to produce a specific amount of moisture in the reactor and then, with the escape valve closed, moisture or its mixture with oxygen or hydrogen is supplied from the reactor to the semiconductor manufacturing line at a set flow rate.
Yet another embodiment of the present invention provides a process for supplying moisture at a low flow rate for use in a moisture generator in which hydrogen and oxygen are fed into a reactor provided with a platinum coat on the wall in the interior space, enhanced in reactivity by the platinum catalytic action and caused to instantaneously react with each other at a temperature lower than the ignition point to produce moisture without undergoing combustion at a high temperature, characterized in that an escape pipe provided with an escape valve is branched out from the outlet side of the flow controller for control of the flow of hydrogen to the reactor and that while oxygen is being supplied at a set flow rate, the escape valve is first opened and hydrogen is fed, thus lowering the pressure on the secondary side of the flow controller, and then the escaped valve is closed to feed hydrogen into the reactor, whereby moisture or a mixture of moisture with oxygen is supplied form the reactor to the semiconductor manufacturing line at a set flow rate.
Finally, another embodiment of the present invention provides a process for supplying moisture at a low flow rate for use in a moisture generator of the same construction as in the fifth embodiment, wherein an escape pipe provided with an escape valve is branched off from the outlet side of a flow controller for control of the flow rate of oxygen flowing to the reactor, and wherein while hydrogen is being supplied at a set flow rate, the escape valve is first opened and oxygen is fed, thus lowering the pressure on the secondary side of the flow controller and then the escape valve is closed to feed oxygen into the reactor, whereby moisture or a mixture of moisture with hydrogen is supplied from the reactor to the semiconductor manufacturing line at a set flow rate.